In this case it has to with our peculiar imperial measurements. 12oz is 1/3 of a kilogram, which happens to be a common container size throughout the rest of the world. This is just standardization, nothing more.
Metrics don't do thirds. The 12 oz is the original, ever since Libby's of Chicago canned beef in the 1870s. And as 12oz meat was the daily soldier's ration, the rest is history.
Also bread 12, 24 oz loaves